\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath, amssymb}
\begin{document}
\(f: I \to \mathbb{R}
\\
x_1, x_2 \in I\)
\end{document}
When I type this in LaTeX, the two rows don't align corectly, they start at different points. Why?
The misalignment is caused by the usual paragraph indentation. \( starts an inline math, similar to normal text starting a pagraph.
With standard settings a paragraph is indented, i.e., the first line is indented by initial horizontal space (of width \parindent), whereas the following lines are unindented.
See Remove Indentation for a Single Paragraph how to suppress the indentation for some paragraphs or use a package like "parskip" to change from indentation to vertical skip between two paragraphs.
Use display math, e.g., \[ or one of the math environments, e.g., equation or align, if you want to get your formulas in separate lines and aligned.
\documentclass{article}
\usepackage{amsmath, amssymb}
\begin{document}
\begin{align}
f: I \to \mathbb{R}
\
x_1, x_2 \in I
\end{align}
\end{document}
(Avoid using \\ to start a new line in a normal paragraph. Use \\ only within environments in which it is defined with the meaning to start a new line entry. Within a normal text paragraph you will get an "underfull hbox" warning message. A new paragraph is started at a new line ìf you input an empty line or a \par in your document file, usually with the first line indented.)
Is it the alignment in math mode searched?
\documentclass[12pt]{article}
\usepackage{mathtools, amssymb}
\begin{document}
(\begin{array}{ll}
f\colon I \to \mathbb{R} &\
x_1, x_2 \in I
\end{array})
\end{document}
